1. Field of the Invention
This invention relates to an improved full-function brake valve for use in fluid braking systems used on the semi-trailers of highway tractor-trailer vehicles, and in particular, tandem trailer braking systems.
2. Discussion of the Prior Art
To meet all of the emergency and service requirements for highway tractor-trailer combinations, while meeting all of the pertinent governmental regulations, including Federal Regulation FMVSS 121 (Docket 90-3, Notice 2), the trucking industry has resorted to a number of trailer brake valve arrangements which have led to complexity, high costs, installation and maintenance difficulties and related problems.
FMVSS 121 is an important safety standard that applies to highway tractor-trailer vehicles; it requires that the spring brakes, which are spring loaded to engage the brake drums when the braking system air pressure drops below the operating pressure necessary to operate the service brakes, become engaged within certain specified time periods. This provides emergency braking when the service brakes become inoperative.
In a typical semi-trailer braking system, a spring brake control valve may be employed to supply pressurized air to the spring brake chambers to release the spring brakes, and also to exhaust air from the spring brake chambers when the spring brakes are to be again applied. The brake system, however, must be designed so that the spring brakes are not released prematurely, that is, prior to pressurization of the reservoir for operating the service brakes. A full-function valve operative to meet these desirable functions has been disclosed in U.S. Pat. No. 5,417,479, issued May 23, 1995, in the names of Steven D. Wallestad and Robert L. Koelzer, and entitled: "Full-Function Valve For Heavy Duty Semi-Trailer Brake System", the disclosure of which is hereby incorporated by reference.
It has been found that where a tandem trailer is employed, i.e., where there are two (or more) trailers to be towed by a tractor, that an added relay valve is needed to boost or enhance the control air signal arriving at the first trailer so that the control air signal is provided to the second trailer at about the same signal strength as it arrives at the first trailer. This relay valve is typically incorporated into the control air line after connection to the control air relay valve that operates the service brakes in the first trailer. In the absence of such an added relay valve, the control air signal will be seriously degraded when it reaches the second trailer. This can lead to weak or uneven application of the brakes, potentially causing uneven brake wear, poor braking stability, and even potential vehicle jackknifing.
It would be desirable if the signal boosting function of the relay valves used heretofore to boost the control air signal could be incorporated into a single full-function valve that also includes a relay valve for activating the service brakes and a spring brake control valve for controlling release and engagement of the spring brakes.